1. Field of the Invention
The present invention is related to diesel engine exhaust systems and methods for reducing NOx emissions.
2. Background Art
Numerous system concepts have been developed for optimizing the efficiency and operability of NOx storage catalysts such as lean NOx traps (“LNTs”). Of these, many have also included the combination of LNTs with selective reduction catalysts (SCR). These system concepts take advantage of the ability of LNTs to store NOx and release ammonia (NH3) periodically when the LNT is regenerated under rich conditions. This NH3 is then stored within a downstream SCR catalyst where it is subsequently available for reaction with NOx which slips through the upstream LNT during lean operation.
The key element in this type of exhaust system is having a SCR positioned downstream from an LNT to store ammonia on the SCR. Since the SCR only provides NOx reduction under lean condition and NH3 is only released by the LNT under rich conditions, NOx reduction by the SCR catalyst is limited to conditions where NH3 storage on the SCR catalyst is significant. Because typical SCR catalysts are only able to store significant quantities of NH3 below ˜400 C, this means that the SCR catalyst can only complement the NOx performance of the LNT significantly below this temperature. It is possible to separate the SCR catalyst from the LNT by placing the SCR catalyst in a separate converter can. Such a scheme is capable of broadening the effective temperature range of SCR complementation. However, packaging limitations on many applications precludes such separate packaging from being utilized in practice.
Accordingly, there is a need for improved engine exhaust systems and in particular diesel exhaust systems with a greater operating temperature range.